A verbal is a verb that acts like a different part of speech. Infinitives, gerunds, and participles are all types of verbals. An infinitive is a verbal consisting of to + a verb, and it acts like a subject, direct object, subject complement, adjective, or adverb in a sentence.
HeadOn is the brand name of a topical product claimed to relieve headaches. It achieved widespread notoriety in 2006 as a result of a repetitive commercial, consisting only of the tagline HeadOn. Apply directly to the forehead, stated three times in succession.
Headcanon. (also written as head canon, but it's really one word.). An idea, belief, or aspect of a story that is not mentioned in the media itself, but is accepted by either the media consumer themselves or the fandom in general. If it is confirmed by the creator of the story, it becomes canon.